Magnetic field and electrical current sensors play an important role in both military and civilian applications. Fiber optic based sensors offer many necessary and attractive benefits that have been driving their development for several decades. These benefits include immunity to electromagnetic interference (EMI), excellent electrical isolation, compatibility with modern communication networks, multiplexibility, compactness, light weight and an inherently explosion proof nature. Militarily useful attributes include resistance to lightning and nuclear blast-induced EMI, light weight for flight systems, low electronic emissions (from the sensors) and explosion proof nature for use with fuels. Potential commercial uses parallel those of the military and may be even more extensive. Tachometry, magnetometry, commercial aircraft instrumentation and petrochemicals are potential markets.
Although fiber optic magnetic field and/or electrical current sensors have been applied to a few markets, notably high voltage current sensing in the power industry, their potential advantages have sometimes been outweighed by their expense, size and lack of physical robustness. It is suitable to divide sensors according to the magneto-optical material utilized. The cheapest material is optical fiber itself. One drawback of such optical fiber sensors is the very small Faraday activity of standard optical glass. Many turns of fiber around the conductor wire or flux path helps to increase the fiber-to-magnetic field interaction length, although birefringence also accumulates per unit length and limits the sensitivity of such sensors. The higher Faraday rotation glasses are utilized in bulk form in the sensors, using multiple total internal reflections (see, e.g., Y. N. Ning et al in “Recent progress in optical current sensing techniques”, Review of Scientific Instruments 66 (5), May 1995, pp. 3097–3111). However, such sensors generally require precise surface machining and optical alignment. By incorporating birefringence compensation, such sensors can gain up to an order of magnitude in resolution. See e.g., N. E. Fisher & D. A. Jackson, Meas. Sci. Technol. 7 (1996)p 1099–1102, but mechanical and environmental stability, complicated installation and higher cost make them impractical.
Other sensors utilize high Faraday-activity materials such as magnetic garnets. See, for example Rochford K. B., Rose A. H., Deeter M. N., Day G. W. “Faraday effect current sensor with improved sensitivity-bandwidth product.” Optics Letters, vol. 19, (no. 22), November 1994, p. 1903 and Day G. W., Deeter M. N., Rose A. H.; Rochford K. B. “Faraday effect sensors for magnetic field and electric current.” Proceedings of the SPIE—The International Society for Optical Engineering, vol. 2292, (Fiber Optic and Laser Sensors XII, San Diego, Calif., USA, 25–27 Jul. 1994.) 1994. p. 42. Such sensors generally exhibit high sensitivity and accuracy, but they can have limited dynamic range due to saturation of magnetic garnets and are also assembled optical components that can become misaligned or damaged with rough use. Another disadvantage is the relatively high temperature dependence of Faraday rotation in magnetic garnets. See e.g., Kamada O., Minemoto H., Itoh N., “Magnetooptical Properties Of (BiGdY)3Fe5O12 For Optical Magnetic-field Sensors” Journal Of Applied Physics, 75: (10) 6801–6803, Part 2B May 1994.
The Faraday effect causes the polarization of linearly polarized light to rotate as it travels through a medium if a magnetic field is present parallel to the direction of propagation of the light. The magnitude of this rotation is different for different materials and in general is proportional to the strength of the applied magnetic field in the direction parallel to the propagation of the light, the length of propagation of the light through the material, and a characteristic of each material known as a Verdet constant (for paramagnetic materials such as, for example, glasses) or a Faraday constant (for ferromagnetic materials such as, for example, iron garnets). The change in the material properties due to the magnetic field is called circular birefringence. More specifically, when a material having a Verdet constant V is exposed to a magnetic field H and, due to this field, gains a magnetization projection onto the direction of the light propagation M, a linearly polarized light beam passing through the material along an optical path l has its polarization azimuth rotated by an amount
      Φ    F    =            ∫      l        ⁢          VM      ·                          ⁢                        ⅆ          l                .            Either V or M, if constant over l, may be moved out of integral. In particular, if both V and M are constant over l, the equation set forth above simplifies to ΦF=VM·l. By transmitting polarized light through the material, exposing the material to a magnetic field and measuring the change in polarization azimuth, one can determine the strength of the magnetic field in the direction of propagation of the light through the material. If an electrical current causes the field, the field and current directions are normal to each other. Optical sensors are also isolated and safe to use under high currents and voltages. The most frequently used are a method by Malus (see e.g., Rochford K. B.; Rose A. H.; Deeter M. N.; Day G. W. “Faraday effect current sensor with improved sensitivity-bandwidth product.” Optics Letters, vol. 19, (no. 22), November 1994. p. 1903; Day G. W.; Deeter M. N.; Rose A. H.; Rochford K. B. “Faraday effect sensors for magnetic field and electric current”, Proceedings of the SPIE—The International Society for Optical Engineering, vol. 2292, (Fiber Optic and Laser Sensors XII, San Diego, Calif., USA, 25 –27 Jul. 1994 p. 42) and a Sagnac interferometer method (see e.g., J. Blake et al. IEEE Transactions on Power Delivery, Vol. 11, No. 1 January 1996; Moon Fuk Chan; Guansan Chen; Demokan M. S.; Hwa Yaw Tam “Optimal sensing of current based on an extrinsic Sagnac interferometer configuration”, Optics and Lasers in Engineering, vol. 30, (no. 1), July 1998. p. 17). The former is simpler to build and less expensive, while the latter is generally more stable.
The Faraday effect is a non-reciprocal effect, which means that light passing through a material exhibiting this phenomenon will pick up an angle of rotation of polarization that is independent of the direction in which the light is traveling. Because of this non-reciprocity, multiple passes through the sensing element increase the magnitude of the polarization rotation and thereby enhance the sensitivity of the sensor proportionally. As an example, a current sensor using multiple passes is described by Ning, et al [Y. N. Ning et al in “Recent progress in optical current sensing techniques”, Review of Scientific Instruments 66 (5), May 1995, pp. 3097–3111]. In this arrangement, a bulk glass exhibiting a high Faraday effect had a hole drilled through it to receive a wire carrying a current. Light was introduced into the bulk glass and reflected several times while going around the hole. The peculiar geometry of this detector limits its usefulness to detection of current flowing through a wire or to a narrow range of very similar uses. Another disadvantage was the necessity of breaking the power line during installation and the sensitivity of the sensor to mechanical noise because of the precise fiber-to-bulk glass alignment needed.
From another point of view, fiber and/or waveguide Bragg gratings have been known in the art for a long time as a sensor element for strain, temperature and pressure. Fiber-optic Bragg gratings (FBG) are known as periodic modulations of the refractive index in the core and/or cladding of an optical fiber, while waveguide Bragg gratings are usually introduced as a periodic variation in thickness and/or surface profile of one or more layers or thin films that the waveguide comprises. When the grating pitch of the Bragg grating coincides with half the wavelength of the light, the reflection conditions of the first order are satisfied.
If magneto-optical material is placed inside the optical resonator (Fabry-Perot [FP] cavity or multi-layer reflector stack, similar to narrow bandpass filter) the polarization rotation exhibits enhancement proportional to the squared quality of the resonator, which is defined by the reflectivity of the mirrors (or multi-layer reflectors). For instance, multilayer stacks of thin films, made like standard interference filters and air gaps between parallel mirrors are both manifestations of FP resonators, or etalons. A multilayer stack concept was demonstrated [Inoue et al. U.S. Pat. No. 6,262,949 Jul. 17, 2001] by fabricating alternating layers of SiO2 and YIG by sputtering. Even though sputtered YIG is not as good a quality as LPE (Liquid-Phase Epitaxy)-grown YIG, the polarization rotation exhibited enhancement proportional to the squared quality factor of the resonator. (The quality factor is defined by the reflectivity of the mirrors and absorption of light in YIG). In another example, in this case a gas-filled gap between parallel mirrors, the magneto-optical activity of certain gases [Vallet M., Bretenaker F., Le Floch A., Le Naour R., Oger M. “The Malus Fabry-Perot interferometer,” Optics Communications, vol. 168, (no.5–6), September 1999. p. 423] has been enhanced by 6 orders of magnitude with the use of such a resonator. This amplification of Faraday rotation can be explained in terms of the longer effective length that a photon travels though the resonator back and forth between mirrors before leaving the resonator. This means that by constructing the cavity with very high reflectivity mirrors, one can obtain an interaction length between the light and the 1 cm magneto-optical material on order equivalent to 10 km (and even more if said material is sufficiently transparent for these purposes).
The YIG/GGG (YIG layer serves as a waveguiding layer and GGG substrate serves as a waveguide substrate) waveguide realizations of such an approach for integrated optical isolators have been theoretically analyzed recently in [M. J. Steel et al., J. of Lightwave Technology, Vol. 18, No. 9, September 2000, pp. 1289, 1297]. Neither different birefringence effects, which are internal for such a waveguide design and are of importance, nor magnetic field and/or electrical current detection have been analyzed in these publications.
In fiber, one can construct high quality optical resonator structures more simply and cheaply than in volume optics or by any method mentioned in any reference so far. To do this, one can write a phase shifted Bragg grating into the fiber.
A preferred exemplary embodiment of the present invention makes use of these principles to provide a new fiber-optic magnetic field or electrical current sensor and associated system that can provide increases in accuracy, resolution and environmental stability. Briefly, the exemplary and illustrative design is based on a phase-shifted fiber or waveguide Bragg grating in which a Fabry-Perot resonator is formed around the phase shift. When the wavelength of incident light coincides with the wavelength of the Fabry-Perot resonator mode, the magnetic field induced polarization rotation of the waveguided light will be strongly enhanced.
In accordance with one non-limiting preferred exemplary aspect of a preferred illustrative embodiment of the invention, an apparatus for detecting a magnitude of a physical condition includes a light source, a magneto-optic Faraday effect sensing element comprising a fiber phase-shifted Bragg grating, first and second polarizers, and a detector. In this illustrative arrangement, the first polarizer is disposed between the light source and the magneto-optic Faraday effect sensing element, and the second polarizer (analyzer) is disposed in an optical path after the magneto-optic Faraday effect sensing element. The second polarizer detects the appearance of polarization state different from the one transmitted through the first polarizer. A detector optically coupled to the second polarizer responds to this detected polarization state.
In accordance with another non-limiting aspect, the light source may comprise a tunable laser such as, for example, a tunable VCSEL [C. J. Chang-Hasnain, IEEE J. on Selected Topics in Quantum Electronics, V. 6(6), November/December 2000; J. S. Harris, Jr., IEEE J. on Selected Topics in Quantum Electronics, V. 6(6), November/December 2000].
In accordance with yet another aspect, the light source may comprise a wideband light source, and the arrangement may comprise a circulator and fiber Bragg gratings or other wavelength filters such as for example Fabry-Perot filters whose filtering reflection feature coincides with the Bragg grating feature of the sensing element.
In accordance with yet another aspect, the light source may comprise a broadband light source such as for example a light emitting diode, a super luminescent diode or a lamp.
In accordance with yet another aspect, the detector may comprise a semiconductor photo diode, or a balanced photodetector. The balanced photodetector may comprise, for example, two photodiodes with a polarization splitter to detect opposite polarization components of the transmitted through the sensing element light.
In accordance with yet another non-limiting aspect, the physical condition being measured may comprise a magnetic field, a current flowing through an electrical conductor, or the like.
In accordance with yet another aspect, a phase-shifted fiber Bragg grating is written into the fiber. This fiber Bragg grating (which may be written into the fiber using ultraviolet laser radiation) may be written into a communication single-mode fiber with symmetrical core (low-birefringent) or a polarization-maintaining single-mode fiber with a highly asymmetric core (high-birefringent).
In accordance with yet another non-limiting aspect, the phase-shifted fiber Bragg grating may comprise a constant-period Bragg grating, or it could be constructed from two or more superimposed phase-shifted Bragg gratings to compensate the birefringence.
In accordance with yet another aspect, the phase-shifted fiber Bragg grating incorporates at least one phase shift.
In accordance with yet another aspect, the exemplary sensing arrangement may include an environmental effects compensation feedback arrangement together with a tunable laser such as, for example, a tunable VCSEL.
In accordance with yet another aspect, the magneto-optic Faraday effect sensing element may include flux concentrators that increase the value of the magnetic field at the position of the phase shift(s).
In accordance with yet another aspect, a waveguide may be included that is constructed so the waveguide mode is at least partially localized in the magneto-optically active material. The magneto-optically active material may, for example, have an in-plane magnetization anisotropy. Another hard magnetic material layer may be disposed such that the in-plane hard axis of the hard magnetic material layer lies in the plane of incidence of light and is collinear to an external magnetic field to be measured. A hard magnetic material layer may produce uniform bias magnetic field in the plane of the Faraday-active magnetic material.
In accordance with another aspect of a non-limiting illustrative embodiment, a hard magnet is placed in the vicinity of the sensing element such that the magnetic field produced by the hard magnet is uniform in the Faraday-active magnetic material and lies in the plane of the Faraday-active material. In accordance with this aspect, the direction of the magnetic field produced by at least one hard magnet is perpendicular to an external magnetic field to be measured. The hard magnet thus provides a uniform bias magnetic field in the plane of the Faraday-active magnetic material.
In accordance with yet another aspect of a non-limiting illustrative arrangement, the Faraday-active magnetic material may comprise a magnetic garnet single crystal thin film. Alternatively, the Faraday-active magnetic material may be constructed from two or more layers of magnetic garnet single crystal thin films with different compositions and magneto-optical properties. The Faraday-active magnetic material may be any other transparent material with Faraday activity sufficiently high for its purpose.
The following are additional advantageous features and advantages provided by the non-limiting illustrative exemplary embodiments:                Phase shifted fiber Bragg grating magnetic field or current sensor.        Multiple phase-shifted fiber Bragg grating magnetic field or current sensor.        Superimposed phase-shifted fiber Bragg grating magnetic field or current sensor.        Phase shifted waveguide Bragg grating magnetic field or current sensor.        Multiple phase-shifted waveguide Bragg grating magnetic field or current sensor.        Superimposed phase-shifted waveguide Bragg grating magnetic field or current sensor (the important design for birefringence compensation).        Tunable VCSEL based read-out method.        Circulator+number of optical filter based read-out scheme.        